clear out class variable in Python [duplicate]

Question

I have a class and created its instances a and b. I think both instances points same address.

For example,

class Foo:
    counter = 0
    def increment():
        counter += 1

a = Foo()
b = Foo()
for i in range(10):
    a.increment()
    b.increment()
    aa = a.counter
    bb = b.counter
    print(aa)
    print(bb)

I expected that aa = 10 and bb = 10. However bb = 20 at this point. What am I missing?

Your counter variable is a class variable instead of an instance variable. You would access instance variables using self. docs.python.org/2/tutorial/… — havanagrawal
– havanagrawal, Commented Mar 21, 2019 at 20:14
your code doesn't work: increment isn't a class method and counter should be self.counter — Jean-François Fabre
– Jean-François Fabre ♦, Commented Mar 21, 2019 at 20:15

blhsing · Accepted Answer · 2019-03-21 20:14:16Z

1

If you intend to make the counter specific to each instance you should make it an instance variable and initialize it in the __init__ method instead:

class Foo:
    def __init__(self):
        self.counter = 0
    def increment(self):
        self.counter += 1

answered Mar 21, 2019 at 20:14

blhsing

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1 Comment

Jean-François Fabre Over a year ago

I think we can call that a classic duplicate and avoid answering this.

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